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Prior posts have noted the impact of irrigated agriculture on regional temperature increases. However, the influence of water vapor is also a key factor in understanding the rate of change in global temperatures.
Researchers recently noted that a sudden and unexplained drop in the amount of water vapor present high in the atmosphere almost a decade ago has substantially slowed the rate of warming at Earth's surface in recent years.
In late 2000 and early 2001, concentrations of water vapor in a narrow slice of the lower stratosphere dropped by 0.5 parts per million, or about 10 percent, and have remained relatively stable since then. Because the decline was noted by several types of instruments, including some on satellites and others lofted on balloons, the sharp decrease is presumed to be real. Because water vapor is a powerful greenhouse gas, the decline has slowed the increase of global temperatures.
Although the cause of the decline is not understood, modeling by the researchers suggests that the decrease in water vapor concentrations in the lower stratosphere has slowed down average global warming. The rate of increase in the average global surface temperature from 2000 to 2009 was about 25 percent lower than it otherwise would have been, the researchers report. The team's analyses suggest that average global surface temperatures rose only 0.1 degrees Celsius during that period, rather than the 0.14 degree increase expected because of increases in other greenhouse gases.
The researchers speculate that the amount of water vapor gradually rising into the stratosphere at tropical latitudes has decreased, possibly due to a shift in global patterns of sea-surface temperatures that influence rates of evaporation and water vapor movement.
The study can be found at http://www.sciencemag.org/cgi/content/short/327/5970/1219.